Little is known about the development of abnormal repetitive behaviors (e.g., stereotypies, rituals) in children at risk for autism and related neurodevelopmental disorders. This lack of knowledge precludes effective early intervention and prevention strategies. Appropriate animal models could provide a wealth of information about the developmental characteristics of such behavior, particularly the neurobiological mechanisms mediating the transition to persistent, fixed, and habitual behavior. Thus, the overall goal of this project will be to examine the development of repetitive motor behavior in a mouse model and identify the neurobiological mechanisms mediating the emergence and expression of habitual, persistent and inflexible motor behavior. We will test hypotheses that the development of persistent repetitive behavior is associated with alterations in corti co-basal ganglia circuitry. Specifically, we will test hypotheses that alterations in basal ganglia pathways (direct and indirect) (Aim #2) as well as striatal compartments (striosome and matrix) (Aim #3) mediate the transition to persistent repetitive behavior. Pharmacological experiments in Aim #2 will not only aid in identifying neurobiological mechanisms but will also identify potential therapeutic targets for drug development.